1. Field of the Invention
The present invention relates to a photo-sensitive fiber and, more particularly, to a photosensitive fiber in which abnormal transmission losses can be avoided.
2. Description of the Related Art
Optical fiber gratings utilizing photo-sensitive fibers have been known. Such photo-sensitive fibers normally employ a single mode fiber which can be connected to an optical fiber with less insertion loss (see Japanese unexamined patent publication No. H8-290931, "Optical fiber design for strong gratings photo imprinting with radiation mode suppression" Delevaque E., Boj S., etal, 1994, Optical Fiber Conference 1995, PD5-2, "Thermally enhanced ultraviolet photosensitive in GeO.sub.2 and P.sub.2 O.sub.5 doped optical fiber", P. J. Lemaire, A. W. Vengsarker, W. A. Reed and D. J. DiGiovanni, Appl. Phys. Lett., 66(16), Apr. 17, 1995).
A single mode fiber comprises a cladding formed of silicon dioxide (SiO.sub.2) and a core which is formed by adding germanium dioxide (GeO.sub.2) to silicon dioxide to provide a high refractive index. Fluorine may be added to the cladding, and fluorine and diphosphorus pentaoxide (P.sub.2 O.sub.5) may be added to the core.
VAD and MCVD are well known methods for manufacturing such single mode fibers.
A fiber grating is an optical fiber type filter which utilizes a phenomenon that the refractive index of germanium dioxide included in silicon dioxide of an optical fiber is increased when irradiated with ultraviolet light because of defects in the germanium dioxide and in which regions having the increased refractive index are arranged in the core along the optical axis to form a diffraction grating to reflect only beams having a wavelength corresponding to the intervals therebetween.
A photo-sensitive fiber is fabricated by irradiating an optical fiber with ultraviolet light under interference provided using holographic interferometry or masking.
Since the increase in the refractive index of germanium dioxide as a result of irradiation with ultraviolet light is attributable to glass defects therein, when an optical fiber is irradiated with ultraviolet light, a diffraction grating is formed only in the core which includes germanium dioxide. As a result, a region which is irradiated by ultraviolet light and a region which is not irradiated by ultraviolet light have different mode field diameters because the difference between the refractive indices of the core and cladding is different between those regions. Such a mode field mismatch results in mode coupling between light that leaks into and propagates through the cladding and reflected light, causing abnormal transmission losses outside the reflection band.
A known method for suppressing such abnormal transmission losses is to add germanium dioxide also to the cladding such that identical diffraction gratings are formed in both of the core and cladding as a result of irradiation with ultraviolet light. In this case, the amount of germanium dioxide added to the cladding must be the same as that added to the core in order that the gratings formed in the core and cladding are identical.
However, since the cladding must have a refractive index lower than that of the core, a dopant must be added to cancel any increase in the refractive index due to germanium dioxide. Fluorine (F) and boron (B) are known as dopants for decreasing the refractive index.
However, although boron is effective in decreasing the refractive index, its refractive index is increased when irradiated with ultraviolet light as in the case of germanium (Ge). Therefore, the refractive index of the cladding is increased beyond that of the core and hence boron can not be used.
Although fluorine may be added, it is difficult to control the amount to add because its mechanism has not been clarified yet. Known materials used as such an additive include silicon tetrafluoride (SiF.sub.4), sulfur hexafluoride (SF.sub.6) and dicarbon hexafluoride (C.sub.2 F.sub.6). With any of these materials, it is difficult to add fluorine in a sufficient amount to cancel the increase of the refractive index attributable to Ge in the same amount as in the core using conventional methods of manufacture.
While the use of SiF.sub.4 allows a relatively large amount of fluorine to be added, it necessitates modification of the manufacturing apparatus because it is not used in the manufacture of a normal fiber matrix.
While C.sub.2 F.sub.6 is used for dry etching of a starting quartz tube during the manufacture of a fiber matrix using MCVD, it can not be added in an amount to cancel the refractive index of a normal single mode core.
According to the above-described methods for manufacturing a photo-sensitive fiber, Ge must be added to the cladding in the same amount as for the core. In a normal single mode, the core includes approximately 7 mol % Ge. When the same amount of Ge is added to the cladding, it is necessary to add another material which has an effect of decreasing the refractive index by an amount corresponding to the increase in the index attributable to Ge. Further, it must be a material having a refractive index which is not affected by ultraviolet illumination in order that the refractive indices of the core and cladding increase by the same amount after irradiation with ultraviolet light. Materials that satisfy those requirements include fluorine (F).
Phosphorus (P) which is effective in increasing a refractive index is added to the core to form regions having a high refractive index in the core with the amount of Ge added therein decreased compared to the prior art. Thus, it is possible to decrease the amount of Ge required to be added to a cladding inner layer.
A decrease in the amount of Ge added to the cladding results in a decrease in the amount of F required to be added, which allows easy fabrication using a conventional process without any special process.
Fluorine can be added to the cladding using dicarbon hexafluoride which is used for dry etching of a starting quartz tube during the manufacture of a fiber matrix using MCVD. In this case, fabrication can be carried out using conventional manufacturing facility.
Since GeO.sub.2 is added to a core and a cladding in the same amount, the refractive indices of the core and cladding also increase by the same amount when they are irradiated with ultraviolet light. As a result, in a photo-sensitive fiber, the difference between the refractive indices of the core and cladding remains unchanged in regions irradiated by ultraviolet light and non-irradiated regions. This prevents any mismatch of mode field diameters and hence any abnormal loss outside the reflection band.
It is known that the refractive index of phosphorus (P) is also increased when irradiated with ultraviolet light for a long time, although only slightly. Therefore, when ultraviolet illumination must be provided for a long time during the fabrication of a photo-sensitive fiber, the amount of Ge added to the core may be adjusted such that the refractive indices of the core and cladding increase by the same amount after being irradiated by ultraviolet light.
The present invention has been conceived taking such a situation into consideration, and it is an object of the invention to provide a photo-sensitive fiber in which an optical fiber grating capable of avoiding abnormal transmission losses is provided.